This invention relates to dual frequency antennas for reception and/or transmission of electromagnetic signals.
Many telecommunication systems in use today require use of multiple frequencies for proper operation. For example, the Global Positioning System (GPS) implemented by the U.S. Government requires use of two frequencies, 1.227 GHz and 1.575 GHz, in order to compensate for some of the effects of frequency-dependent ionospheric delay on propagation of electromagnetic signals through the ionosphere. For similar reasons, the GLONOSS global positioning and navigation system of the former Soviet Union uses two frequencies, 1.246 GHz and 1.602 GHz. One design of a wireless Local Area Network (LAN) uses three frequency ranges, 0.902-0.928 GHz, 2.400-2.485 GHz and 5.725-5.850 GHz, for control and data transmission purposes. Use of multiple frequencies may require use of multiple antennas, which may create space allocation problems in a transmitter or receiver with a size constraint imposed.
Microstrip antennas, which were first discussed by G. A. Deschamps in "Microstrip Microwave Antennas", Third U.S.A.F. Symposium on Antennas", 1953, offer a possible solution to the size problem for multiple antennas. These antennas have been discussed in more detail by J.Q.Howells in "Microstrip Antennas", I.E.E.E. Trans. on Antennas and Propagation, 1975, pp. 90-93; by R.E.Munson in "Microstrip Antennas", in Antenna Handbook, edited by Skolia, pp. 7-1 to 7-28; and by I. J. Bahl and P. Bhartia, Microstrip Antennas, Artech House, 1984, pp. 1-29 et seq.
A microstrip (ms) antenna, in its simplest form, consists of a thin electromagnetic resonator layer of carefully chosen dimensions, a ground plane, a dielectric layer contiguous to and separating the resonator and the ground plane, and an antenna signal feed connected to the resonator at a carefully chosen position. Microstrip antennas are available as patch antennas, as traveling wave antennas and as slot antennas, depending upon the geometry chosen for the resonator. These types of ms antennas are discussed and contrasted by Bahl and Bhartia, op. cit. A ms antenna offers several advantages relative to conventional antennas: (1) the ms antenna size is quite small, having typical dimensions of the order of 10 cm.times.10 cmx 1 cm; (2) fabrication cost of a ms antenna is low for high volume production; (3) a ms antenna has low scattering cross-section; (4) linear, as well as circular (right hand or left hand) polarization for the radiating waves is available; (5) fed lines are fabricated simultaneously with fabrication of the remainder of the ms antenna; and (6) the choice of operating frequency may be chosen over a broad range from 100 MHz to 50 GHz. However, the ms antenna also has certain disadvantages: (1) the bandwidth for ms antenna operation is usually small, with a typical full width at half maximum (FWHM) of about 10 MHz; (2) an ms antenna has some loss so that gain is limited, usually to 20 dB or less; (3) except for special designs, an ms antenna usually radiates into a half plane and has poor endfire performance; (4) isolation between the feed line and the radiating element is a serious problem; (5) an ms antenna may excite surface waves as well; and (6) an ms antenna has relatively low power handling capability.
Dual frequency ms antennas have been discussed by Munson, op. cit., and by Bahl and Bhartia, op. cit., pp. 69-75, 127-132 and 157-162, and elsewhere in the literature within the last ten years. These dual frequency configurations usually employ a stacked ground plane and first and second patch resonators, spaced apart by two dielectric layers, with the first and second resonators each radiating at distinct resonant frequencies. Each patch resonator requires a separate feed line, and the two frequencies must differ from each other by at least 10-20 percent. The material for, and thickness of, the dielectric layer separating the first and second resonators must be carefully controlled to provide reasonable electromagnetic isolation of the resonators and their associated frequencies.
What is needed is a more compact dual frequency ms antenna that does not require fabrication of two electrically separated resonator regions and for which the dielectric materials and thicknesses used are not so critical in fabrication of the antenna. Preferably, the dual frequency antenna should allow use of a wide range of dielectric materials and should offer improved spatial directivity and FWHM bandwidth for the radiation fields.